Thermoplastic polymers are capable of being formed into cellular materials by a process generally known as solid-state foaming. Unlike the use of an extruder, wherein a thermoplastic polymer is melted to a liquid state into which a blowing agent is injected, the solid-state process does not melt the thermoplastic polymer via an extruder to inject the blowing agent, but instead, the thermoplastic polymer remains solidified during the process of absorbing a gas. Thereafter, the gas-saturated solid-state polymer is converted into a foam by raising the temperature. A solid-state foaming process is illustrated in FIG. 1.
The solid-state foaming process illustrated in FIG. 1, requires a solid-state thermoplastic polymer that has been saturated with gas. The foaming occurs while the polymer remains in the solid state either by a rapid decrease in pressure or through the application of heat. This process differs from extrusion foaming processes because the polymer is not required to be in a molten state when the gas is injected. Generally, at the beginning of the solid-state foaming method, block 100, the polymer is in equilibrium with the surrounding temperature and pressure so that the polymer is unsaturated. In block 102, the thermoplastic polymer is treated at an elevated pressure with an inert gas to cause the thermoplastic polymer to absorb the gas. Suitable gases may include non-reacting gases, such as carbon dioxide or nitrogen. The treatment of the polymer in block 102 may be carried out by placing the polymer in a pressure vessel which is sealed, and then the polymer is exposed to the inert gas at a high pressure. The highly pressurized gas will start to diffuse into the thermoplastic polymer over time, filling the polymer's free intermolecular volume. The gas will continue to saturate the polymer until a suitable gas concentration is reached or until equilibrium is reached. In block 104, the fully saturated or partially saturated polymer is removed from the saturation pressure vessel to an environment of lower pressure so that the polymer becomes thermodynamically unstable, meaning that the polymer is supersaturated with gas that is no longer at equilibrium with the surrounding environment. The polymer then desorbs gas from its surface into the surrounding environment. Desorption of some of the gas from the polymer's exterior surfaces will lead to, upon subsequent heating, the creation of a non-cellular skin at the exterior surfaces due to the lack of gas. Heating of the gas-saturated polymer in block 106 is carried out at a temperature below the melting temperature of the polymer. At a certain temperature, nucleation sites appear in the polymer-gas matrix, and further gas diffusion from the polymer-gas solution surrounding the sites causes the sites to form cells.